This invention relates to multimodal polymer particle compositions having at least two populations of polymer particles that are useful as impact modifiers. This invention also relates to plastics additive polymer particle dispersions having at least two populations of polymer particles that can be formed into a powder. This invention further relates to a process for preparing multimodal polymer particle compositions having at least two populations of polymer particles that are useful as impact modifiers. This invention even further relates to polymeric compositions that include a polymeric component and one or more impact modifiers prepared from multimodal polymer particle compositions.
As used herein, the term “multimodal polymer particle composition” refers to a composition having at least two populations of polymer particles, a “larger mode” and a “smaller mode”, wherein the at least two populations of polymer particles differ in mean particle size by at least 50 percent. Optional additional modes may have mean particle sizes even larger than the “larger mode” as well as smaller than the “small mode”.
Numerous molded articles and films are manufactured from one or more of a variety of polymeric resins. Oftentimes, these resins by themselves are brittle and do not possess suitable impact strength required by the end use for which they are made. To overcome their shortcomings, resins, especially poly(vinyl chloride), hereinafter “PVC”, are generally blended with plastics additives that improve impact strength. Such plastics additives are typically known in the industry as impact modifiers and are often supplied in a powder form.
Many impact modifiers are based on polymer particles that contain a majority amount of a soft rubbery polymer phase (e.g., core) surrounded by a hard polymer phase (e.g., shell), herein referred to as “core-shell” polymer particles. While the rubbery core polymer contributes to the toughening effect of the impact modifier, it is inherently soft and sticky and generally cannot be isolated neatly as a dry powder. The hard shell therefore surrounds the sticky core polymer and enables one to isolate the core-shell polymer particles as a dry powder.
For improving process economics and properties, polymer particles can be prepared using emulsion polymerization techniques to create a bimodal distribution of particle sizes (e.g., a “smaller mode” and a “larger mode”). Such bimodal distributions enable the preparation of polymer particle dispersions at high solids (e.g., typically at least 50 weight percent) at practicable process viscosities (e.g., typically below about 2000 centipoise, “cPs”).
Spray drying is an economical, safe and desirable means of isolating dispersions of polymer particles as free-flowing powders. During this process, an aqueous dispersion of polymer particles is atomized in a chamber containing heated air, water is removed, and the polymer particles are aggregated into dry powder particles.
While spray-drying is useful for preparing dry powders from non-rubbery (high Tg) polymer particle dispersions that are low in viscosity, there are several problems associated with spray drying polymer particle dispersions having high solids, high viscosities, and which are composed primarily of a soft, rubbery polymer phase. These problems include: (1) sticking of the particles to the chamber walls of the spray dryer; (2) bridging of the particles over conveying lines entrances; and (3) unacceptable powder flow characterized by aggregation, clumping, and flow interruptions.
It has previously been unrealizable to prepare and dry high solids rubber-containing multimodal polymer particle dispersions having a “smaller mode” mean particle size that exceeds 200 nm.
Canadian Patent 1,256,645 discloses bimodal polymer particle dispersions having up to 65 percent solids weight fraction. These dispersions, which are prepared by various emulsion polymerization techniques, are disclosed to reduce energy requirements and to achieve an increase in productivity in the production of powders by spray drying. While the disclosed polymer particle dispersions are composed in large measure of acrylic monomers and have a glass transition temperature of at least 45° C., this patent does not address the aforementioned problems associated with spray drying polymer particles composed of a majority of a soft, rubbery polymer phase.
A problem addressed by the present invention is to provide multimodal polymer particle compositions having a soft rubber majority phase, and having a smaller mode of average particle size of greater than 200 nm, which have practicable process viscosities at solids weight fractions of at least 50 weight percent. The term “practicable process viscosities” refers to the ability to prepare polymer particle dispersions by emulsion polymerization techniques and to dry (e.g., by spray drying) such dispersions into a powder. We have now discovered that certain multimodal polymer particle compositions can be readily prepared by emulsion polymerization and dried into a powder.
We have surprisingly discovered that such multimodal polymer particle compositions are provided as polymer particles that contain two or more populations (“modes”) of polymer particles that vary in mean particle size, wherein the mean particle sizes of the two populations vary by at least 50 percent, and the smaller mode has a mean particle diameter of greater than 200 nm. In addition, we have surprisingly discovered that multimodal polymer particle dispersions containing up to 95 weight percent total rubbery component, based on total polymer particle weight, can be readily spray-dried to compact free powders when the polymer particles are prepared via a “gradual addition” (as opposed to a “shot monomer addition”) free-radical emulsion polymerization addition. We have also discovered that these new multimodal polymer particles are useful as additives for plastics, and are especially useful as impact modifiers when they contain at least 70 weight percent of a rubbery component.